At present, high-voltage transmission lines are distinguished into two types: overhead lines and underground cables. Along with the development of urbanization, however, people try to change the overhead lines to the underground cables in more and more cities. Because the cables are under ground and hidden, and they are taken on heavy power transmission task, how the underground cables are ensured to be safe and reliable has been paid attention by the power supply department in each city.
The underground cables always need connect two cable segments together during building and mounting. Due to the technical limitation, contact resistance is produced unavoidably. For cables that undertake high-current transmission, the joints are heated easily due to the contact resistance, so insulating layers are aging rapidly, and arc-over accident may be occurred before the design service life, which could induce fire hazard in serious event. This type of accident is unpredictable, and it always results in extensive power blackout, which can be dangerous to safety in production, and it can make large economic losses.
Because the change of the cable joint's temperature is continuous and slow, there is long time from the joint overheated to the accident happened to take measures for avoiding the accident, so that the lines' safety can be ensured. For this purpose, it is necessary to set up a real-time on-line monitoring system to record the real and continuous temperatures of the underground cable joints, which not only can dynamically monitor the temperature change of the joints, but also can offer the historical data comparison and evaluate and analyze the condition. So the parametric variation and the prewarning information can be offered on time before the accident happens, and the relevant departments are warned to check the lines' safety and take necessary measures for prevention. The loss of the cable accident can be mostly minimized. Now, the domestic technicians attempt to implant optical fibers into the cables to utilize the physical feature that the refractive index of the optical fiber changes when the temperature changes to research on-line temperature detect of the cables. But in practice, many of non-fiber cables are utilized, so the solution of implanting the optical fibers into the cables can not radically resolve the problem of on-line temperature detect of the cables. Secondly, according to the practical structure of implanting the optical fibers into the cables and the usage condition, when using the optical fibers to measure temperatures, the measured values are the temperatures of the parts where the optical fibers are implanted into the cable structural layer, but not the real temperatures of the conductor cores of the cables. Thirdly, the fiber-implanted cables experience large traction force and stretch when laid down underground, and flexural deformation occurs at the same time, so the optical fibers are liable to break off and lose all the functions. So, the application of the on-line temperature measurement technique realized by implanting the optical fibers into the cables is restricted, and the measurement technique can not radically meet the requirements of accurately measuring temperatures of the intermediate cable joints.